Screw rotor machine



9, 1966 L. B. SCHIBBYE 3,265,292

SCREW ROTOR MACHINE Filed Jan. 5, 1966 5 Sheets-Sheet 1 Fig.1

INVENTUR W CLM ATTORNEY Aug. 9, 1966 1.. B. SCHIBBYE SCREW ROTOR MACHINE 5 Sheets-Sheet 2 Filed Jan. 5, 1966 ATTORNEY 9, 1966 L. B. SCHIBBYE 3,265,292

SCREW ROTOR MACHINE Filed Jan, 5, 1966 5 Sheets-Sheet 4 F i g .23

I NVEN TOR 7 ATTORNEY Aug. 9, 1966 B. SCHIBBYE 3,265,292

SCREW ROTOR MACHINE Filed Jan 5, 1966 5 Sheets-Sheet 5 Fig.3

(O LO INVENTOR ATTDRNE Y United States Patent M 3,265,292 SCREW RGTOR MACHENE Lauritz Benedictus Schihbye, Saitsjo-Duvnas, Sweden, assignor to Svenska Rotor Maskiner Airtieholag, Nacka, Sweden, a corporation of Sweden Filed Jan. 3, 1966, Ser. No. 518,394 Claims priority, application Sweden, Jan. 13, 1965, 373/65 13 Claims. (Cl. 230-153) The present invention relates to a screw rotor machine for an elastic working fluid which comprises co-axial rotors journalled in a housing in parallel groups and provided with helical lands and intervening grooves having wrap angles of less than 360 C. In such a machine one rotor in a group cooperates with a rotor in another group within each set of axially equally located rotors of the different groups. At least in one set one of said cooperating rotors is of male rotor type and the other of female rotor type which means that in a plane transverse to the rotor axes the lands and grooves of the male rotor are located at least mainly outside the pitch circle of the rotor and provided with substantially convex flanks and that the lands and grooves of the female rotor are located at least mainly inside the pitch circle of the rotor and provided with substantially concave flanks. The housing has at least one working space provided with low pressure and high pressure ports and barrel and end walls which sealingly enclose the rotors provided therein. When the rotors rotate they cooperate with each other and with the walls of the working space such -that there are formed Chevron-shaped chambers each of which comprises communicating portions a groove in each rotor. The base ends of these chambers lie in a stationary plane transverse to the rotor axes and located atthe high pressure port while the apices lie at the intermeshes between cooperating lands and thus move relatively to the stationary transverse plane during the rotation of the rotors so that the volume of each Chevron-shaped chamber varies.

Machines of this type are mainly intended for use as multistage compressors but also compressors and expanders and compressors directly driven by expanders may be built in similar manner. It is also possible to provide compressors and expanders with synchronizing gears in this manner, said synchronizing gears being regarded as rotors. Even though in the following part of the specil cation only multistage compressors are described for the sake of simplicity the invention is thus not limited to such compressors.

In for instance US. Patent No. 2,659,239 it has been suggested to make multistage compressors of the abovementioned type which are so designed that at least in one group of coaxial rotors one rotor is provided with a stud while another rotor is provided with a central bore which surrounds said stud. In order to render it possible to adjust the relative angular position of the two rotors the bore has a diameter which is somewhat larger than the corresponding stud diameter. The rotor containing the bore is further provided with a tubular stud concentric with the first mentioned stud so that both studs directly and independently of each other can be connected to a gear wheel of a synchronizing gear connecting the two coaxial rotor groups. However, this rotor design involves that each rotor can be journalled in the housing only at one end while the other end only rests on the stud of the other rotor. Therefore the rotors undergo strong deflection which is further increased due to the fact that the rotor stud passing through the other rotor has a small moment of inertia. As a consequence the clearances between cooperating rotor and between the rotors and the 3,265,292 Patented August 9, 1966 housing must, for the reason of mechanical reliability in operation, be made so large that they detrimentally influence the efliciency of the machine which involves that the machine will be practically unusable.

Further in for instance US. Patent 2,975,963 there is suggested to make two stage compressors of the type mentioned type mentioned above which are designed such that the different rotors of each coaxial group are manufactured from a common blank the two rotors having identical screw pitch angles and being non-rotatably secured to each other. Even if the rotor profiles are made diiferent in the two stages it is dificult to obtain a volume ratio between the stages of such a value that the pressure ratios in the two stages will be substantially equal which is desirable with regard to the efficiency. In order to obtain pressure ratios as equal as possible the low pressure stage has therefore been made as long as possible, i.e. it has been formed with threads of the largest possible wrap angle while the high pressure stage has been made as short as possible i.e. it has been formed with the smallest possible wrap angle. However, the efficiency of each stage varies with its wrap angle and particularly in the high pressure stage there has in this manner been obtained an efficiency considerably below the efficiency corresponding to the ideal wrap angle. As a consequence the overall efliciency of the compressor has become low. Further, as mentioned above the directions of the threads are the same in both stages which results in that the axial forces acting on the rotors and emanating from the working fluid are added up whereby loading the thrust bearings so heavily that it is diflicult to obtain a suflicient service life for these hearings particularly if they are made as anti-friction bearings which is desirable because such bearings make it possible to use very small clearances in order to obtain a good efliciency.

In for instance US. Patent 3,074,624 there has further been suggested to make two stage compressors of the type mentioned above which are so designed that the mutually coaxial male rotors are non-rotata'bly connected to each other while the mutually coaxial female rotors are rotatable relatively to each other so that the two female rotors rotate independently of each other. This compressor type is therefore similar to the compressor according to US. Patent 2,659,239 but differs from said compressor by the fact that there is no common synchronizing gearing for the two cooperating rotor pairs. It is therefore necessary that the two female rotors are rotatable relatively to each other during operation which necessitates a further increase of the clearance between the stud and the bore so that the deflection will be still greater than in the compressor according to said patent. It has there fore been necessary to journal the different coaxial rotors separately in the end walls of the housing and in the par tition separating the working spaces. This involves a complication and an increase in the piece of the compressor and due to the oil leakage from the bearings in the partition this type of compressor is not usable in practice as a dry compressor, i.e. a compressor in which the working fluid is not mixed with a liquid during the compresslon.

The present invention has for its object to provide a screw rotor machine of the type described in which the rotors are effectively journalled in the housing causing the smallest possible deflection, the different coaxial rotors may be designed with different screw pitch angles and with opposite thread directions and in which the different coaxial rotors in a simple manner may be adjusted and locked in relation to each other. The invention thus eliminates the drawbacks of the above mentioned previously suggested compress-or types.

The details of the invention will be apparent from the following description of a suitable embodiment of a two stage compressor according to the invention. Here FIG. 1 is a longitudinal section of a compressor along line 11 in FIG. 2,

FIG. 2 is a transverse section along line 2--2 in FIG. 3,

FIG. 2a shows a detail of FIG. 2 on a larger scale,

FIG. 3 is a horizontal section along line 3-3 in FIG. 2 and FIG. 3a shows a detail of FIG. 3 on a larger scale.

The compressor shown in the drawing comprises a housing composed of a first end plate 10, a first casing 12, and intermediate plate 14, asecond casing 16 and a second end plate 18. The first and second casings 12. and 16 surround a first working space 20 and a second working space 22, respectively, in the form of two intersecting bores having parallel axes and the bores in the two working spaces being coaxial. The first working space is axially closed by the first end plate and the intermediate plate 14 while the second working space is axially closed by the other end plate 18 and the intermediate plate 14. Further the first working space 20 is provided with a low pressure port 24 and a high pressure port 26 while the second working space 22 is provided with a low pressure port 28 which communicates with the "high pressure port 26 of the first working space through channels 30 in the first casing 12, through openings 32 in the intermediate plate 14 and through channels 34 in the othercasing 16, and with a high pressure port 36. The two high pressure ports 26, 36 are located adjacent to and on either side of the intermediate plate 14 and on the same side of the plane defined by the axes of the bores which form the working spaces 20, 22 so that the axial forces acting on the rotors partially counterbalance each other.

In the first working space 20 are provided a first female rotor 38 provided with six helical lands and intervening grooves of a wrap angle of about 160 said lands and grooves lying mainly inside the pitch circle of .the rotor 38 and having substantialy concave flanks, and a first male rotor 40 provided with four helical lands and intervening grooves of a wrap angle of about 240 said lands and grooves lying mainly outside the pitch circle of the rotor 40 and having substantially convex flanks. The rotors 38, 40 which constitute a first set of cooperating rotors are substantialy shaped with profiles of the type described in US. Patent 2,622,787 and have the same outer diameter and an axial length which is about twice as great as the outer diameter of the rotors. However, the invention is not limited to these numbers of lands or to this profile type or to these ratios between the outer diameters and between the length and the outer diameter but the male and female rotors can with regard to their form be varied in dependence on the actual conditions for each individual machine.

The first female rotor 38 is provided with a first stud 42 which is journalled in a radial bearing 44 of ballbearing type provided in the first end plate 10 and a second stud 46 which extends through the intermediate plate 14 and the second working space 22 and is journalled in a combined thrust and radial bearing 48 of ball-bearing type provided in the second end plate 18 and adapted for axial zero clearance. In a similar manner the first male rotor 40 is provided with a first stud 50 which extends through the first end plate 10 and serves as the drive shaft of the compressor and is journalled in radial bearing 52 of ball-bearing type provided in the first end plate 10, and a second stud 54 which extends through the intermediate plate 14 and the second working space 22 and is journalled in a combined thrust and radial bearing 56 of ball-bearing ty-pe provided in the second end plate 18 and adapted for axial zero clearance. The first working space 20 and the first rotor set comprising the first female rotor 38 and the first male rotor 40 constitute the first stage, i.e. the low pressure stage of the compressor.

The second or high pressure stage of the compressor comprises the second working space 22 and the second set of cooperating rotors including a second female rotor 58 and a second male rotor 60 provided in said second working space. On the whole these rotors are shaped in the same manner as the first female and male rotors 38, 40 but they have greater screw pitch angles and opposite screw thread directions and the ratio between length and outer diameter amounting only to about 0.8 and a pcripheral flank clearance between the rotors 58, 60 which is about half as large as the peripheral flank clearance between the rotors 38, 40 of the first stage. The design of these second rotors can however be varied with the scope of the invention, the only requirement being that coaxial interconnected rotors must have the same pitch diameter in both stages. As the helices of the two rotor sets are oppositely directed it is of greatest importance that the two groups of coaxial rotors are axially fixed relatively to each other and therefore it is absolutely necessary in a machine of this type to use thrust bearings the axial clearances of which are zero as mentioned above.

The second female rotor 58 is provided with a central through bore 62 which surrounds the second stud 46 of the first female rotor 38 and is provided with two internal cylindrical projections 64, 66 which are accurately machined. These projections 64, 66 cooperate with two accurately machined projections 68, 70 on the stud 46. The stud projection 68 located nearest to the intermediate plate 14 has a somewhat larger diameter than the projection 64 in the bore 62 cooperating therewith so that an interference is obtained between these projections 64, 68. The other projection 70 at the free end of the stud 46 has a diameter which is somewhat less than the diameter of the bore projection 64 just mentioned so that at assembly and disassembly the second female rotor 58 can be pushed on to and pulled away, respectively, from the stud 46 without these projections 64, 70 contacting each other, while the diameter of the projection 70 is somewhat larger than the diameter of the cooperating 'bore projection 66 so that an interference is obtained also between these projections 66, 70. Owing to this interference the rotors 38, 58 are firmly interconnected and the stud 46 isstiffened as the composite body consisting of the stud 46 and the second female rotor 58 will be considerably stiffer than the stud 46 alone and therefore the deflection of the rotors isdecreased. In each of the projections 68, 70 of the stud 46 there is an annular groove 72 and 74, respectively. In the stud 46 there are provided two channels 76 and 78 each communicating at one end thereof individually with said grooves 72, and 74, respectively, and at the other end thereof opening at the free end of the stud 46 where the channels are provided with threads 80 and 82, respectively, for connection to a liquid pump not shown. In the stud 46 there is provided a further channel 84 one opening of said channel being located at the outer surface of the stud 46 between the projections 68, '70, thereof while the other opening of the channel 84 is located at the free end of the stud 46 and provided with a thread 86 so that it can be connected to a liquid pump. By pressing liquid in between the cylindrical surfaces of the projections 64, 68 and 66, 70, respectively, and draining liquid leaking out between the projections through the channel 84 the interference between the projections can be eliminated so that the second female rotor 58 can be rotated on the stud 46 of the first female rotor without being subjected to axial forces and their relative positions and particularly their relative angular positions may be adjusted without any risk of deforming the surfaces of the projections 64, 68 and 66, 70, respectively.

In order to pull off the second female rotor 58 pressure liquid is pressed through all three channels 76, 78, 84 in the stud 46 so that the interference between the projections 64, 68 and 66, 78, respectively is eliminated as described above and as a consequence of the difference in diameters between the projections 64 and 66 the second female rotor 58 will be subjected to an axial force tending to move the rotor towards the free end of the stud so that a disassembly can take place without special implements and without any risk of deformation of the projections 64, 66, 68, 70. Further, the second stud 46 of the first female rotor 38 is provided with a shoulder 88 adapted to determine the axial position of the second female rotor 58 and with a nut 98 adapted to press the second female rotor 58 against the shoulder 88 on the stud 46. When the rotors 38, 58 are mounted in the machine the nut is loosened a little in order not to prevent the angular adjustment of the second female rotor 58 on the stud 46.

The second male rotor 60 is provided with a through bore 92 having internal cylindrical projections 94, 96 in the same manner as the second female rotor 58 and this bore 92 surrounds the second stud 54 of the first male rotor 40 which likewise is provided with cylindrical projections 98, 100 having grooves 102, 184 communicating with channels 106, 108 in the stud 54, these channels being provided with threaded connecting means 110, 112 at the free end of the stud. The stud 54 further contains a draining and disassembly channel 114 with a threaded connection 116 and is provided with a shoulder 118 and a nut 120 for holding the second male rotor 68 axially on the stud 54 of the first male rotor. A locking pin 122 is further inserted into a bore in the stud 54 and in the second male rotor 60 which guarantees that the torque transmitting capacity between the stud and the rotor is sufiicient for the considerably greater torque which must be transmitted between the two male rotors as compared with the two female rotors. Naturally such a locking pin can also be provided between the stud 46 of the first female rotor 38 and the second female rotor 58 if such a measure should be necessary for guaranteeing the torque transmission therebetween. However, in a machine of the type shown in which the addendum of the female rotor outside its pitch circle amounts to two percent of the outer diameter and the total pressure increase is from 1 kp./cm. -9 kp./cm. with equal pressure ratios in both stages the power to be transmitted between the stud 54 of the first male rotor and the second male rotor 60 is about thirty times larger than the power transmitted between the second female rotor 58 and the stud 46 of the first female rotor and therefore, normally, it is sufiicient to provide a locking pin only between the male rotors of such a machine. If a locking pin is to be used between the female rotors the relative angular position of these rotors must be adjusted before the bore for the pin is drilled. The nut 12!) serves to lock the locking pin 122 and is tightened into contact with the second male rotor 60 as an angular adjustment can not take place after the insertion of the locking pin 118 in its bore.

In the barrel wall of the second working space 22 there is provided a number of liquid injecting openings 124 located adjacent to the high pressure port 36 and spaced along the line of intersection between the two bores of the second working space 22. The liquid injected therethrough is supplied from a liquid chamber 126 provided in the second end plate 18 and being in communication with an oil pump 128 directly driven by the second stud 46 of the first female rotor 38. The liquid chamber 126 also communicates with a second liquid chamber 134 in the first casing 12 through channels 138 in the second casing and a channel 132 in the intermediate plate 14. Liquid from the second liquid chamber 134 is injected into the first working space 28 through liquid injection openings 136 provided in the barrel wall of the first working space 20. Liquid is further supplied to two annular chambers 140, 142 in the intermediate plate 14 through a channel 138 in said intermediate plate 14 communicating with the channel 132, each of said annular chambers surrounding one of the second studs 46, 54. of the first rotors 38, 48. In the intermediate plate 14 there are further provided sealing means 144, 146

acting along the rotor studs 46, 54 and sealingly separating the annular chambers 140, 142 from the two working spaces 20, 22. Oil is further supplied from the first liquid chamber 126 through a channel 148 to a bearing chamber 150 in the second end plate 18 enclosing the combined thrust and radial bearings 48, 56 and being drained to the low pressure port 28 of the second stage through channels 152. Oil from the second liquid chamber 134 is also supplied through a channel 154 to a bearing chamber 156 in the first end plate 10 enclosing the radial bearings 44, 52 and being drained to the low pressure port 24 of the first stage through channels 158. In this manner the pressure in the bearing chamber 156 in the first end plate 10 is kept lower than the pressure in the bearing chamber 150 in the second end plate 18 resulting in a certain counterbalancing of the thrust forces acting on the rotors 38, 50 and 40, 60, respectively.

The assembly of the compressor described takes place in the following way. The two first rotors 38, 40 are inserted in the compressor housing and the bearings 44, 52 are mounted in their seats in the first end plate 10. The two second rotors 58, are heated to such a temperature that they can readily be pushed on to the corresponding studs 46, 54 on the first rotors 38, 40. The second rotors 58, 60 are mounted on the studs 46, 54 and the second male rotor 60 is mounted in such an angular position relatively to the stud 54 that the lands of the rotor 68 at the rotor end facing the intermediate plate 14 will lie approximately in alignment with the grooves of the first male rotor 40 at the end of this rotor facing the intermediate plate 14 in order to reduce the risk of rotor oscillations and the second rotors 58, 60 are then left to cool down. Thereafter pressure liquid is supplied to the channels 76, 78, 106, 108 from a liquid pump while the channels 84, 114 are kept open so that the interference between the lands 64, 68; 66, 94, 98; 96, 180 is eliminated and the second rotors 58, 60 can be adjusted on the studs 46, 54. If necessary the relative angular position of the two male rotors 40, 61] is then adjusted and the two second rotors 58, 60 are. locked axially against their shoulders 88, 118 by tightening of the nuts 90, 128. The liquid pressure in the channels 76, 78, 106, 108 in the studs 46, 54 is then relieved and the nut for the second male rotor 68 is removed. Thereafter the bore for the locking pin 122 is drilled in the stud 54 and the second male rotor 60 and the locking pin 122 is inserted therein before the nut 128 is again mounted, tightened and locked. The nut 98 for the second female rotor 58 is loosened and only slightly tightened so that the axial position of the rotor 58 on the stud 46 is maintained but so that rotation of the rotor on the stud is not prevented due to axial clamping and the nut 99 is then locked. The thrust bearings 48, 56 which are adjusted for zero clearance are then mounted such that the desired axial clearance between the rotors and the housing is obtained whereafter pressure liquid is again supplied to the channels 76, 78 in the stud 46 of the first female rotor while the channel 84 is kept open. The first rotors 38, 40 are adjusted angularly in their bearings 44, 48, 52, 56 such that flank contact is obtained between the trailing flank of each male rotor land and the leading flank of each female rotor land and the second female rotor 58 is angularly adjusted on the stud 46 of the first female rotor 38 such that a corresponding flank contact is obtained between the second rotors 58, 60 and thereafter the liquid pressure in the channels 76, 78 is relieved. Hereby is obtained that in operation direct flank contact is obtained in the second stage between the leading flank of each male rotor land and the trailing flank of each female rotor land while in the first stage each rotor land will be located in the middle of the corresponding groove of the other rotor without any flank contact therebetween.

In order to disassembly the compressor the thrust bearings 48, 56, the nuts 90, 122, the locking pin 122 and the second end plate 18 are removed and thereafter pressure liquid is supplied to the channels 76, 78, 84, 106, 108, 114 so that the interference between the second rotors 58, 60 and their studs 46, 54 is eliminated and the axial forces arising due to the differences in diameter between the projections 64, 66 and 94, 96, respectively, move the second rotors 58, 60 such that they are disengaged from the studs 46, 54 passing through them.

The invention is of course not limited to the embodiment shown but covers also machines in which the different groups of coaxial rotors are interconnected by means of synchronizing gears. In such machines the clearances are preferably equal in all stages and the angular adjustment of coaxial rotors is carried out in one operation under pressure liquid supply to release the interference while the angular adjustment of the coaxial rotor grooves may be carried out in suitable manner by means of the synchronizing gears in a second operation.

I claim:

1. A screw rotor machine for an elastic working fiuid comprising coaxial rotors journalled in a housing in parallel groups and provided with helical lands and intervening grooves having wrap angles of less than 360, one rotor in a group cooperating with a rotor in another group within each set of axially equally located rotors of the different groups and one of said cooperating rotors at least in one set being of male rotor type and the other of female rotor type, the housing having at least one working space provided with low pressure and high pressure ports and barrel and end walls sealingly enclosing rotors provided therein, one rotor at least in a first coaxial rotor group comprising at least one axially projecting stud, at least one further rotor having a central bore surrounding and connected with said stud by the interference between at least one pair of rotationally symmetrical surfaces located on the stud and in the bore, respectively, said interconnected rotors being provided with means for accurately determining their relative axial position, and at least one channel being provided in one of said interconnected rotors, said channel having an opening located in the barrel surface of the rotor axially between the ends of said further rotor and another opening provided with means for connection to a source of liquid under high pressure, whereby with the rotors standing still liquid can be pressed in 'between the stud and the surrounding further rotor.

Z. A screw rotor machine as defined in claim 1, in which the rotor having at least one axially projecting stud is of female rotor type.

3. A screw rotor machine as defined in claim 1, in which the mutually coaxial rotors cooperating with the rotors of the first coaxial group are also connected to each other by the interference between a stud on one rotor and a bore in another rotor.

4. A screw rotor machine as defined in claim 1, having means for positively coupling the coaxial rotors together.

5. A screw rotor machine as defined in claim 1, in which said channel opening in the barrel wall is located between axially adjacent interference surfaces.

'6. A screw rotor machine as defined in claim 1, in which the axially projecting stud has at least two cylindrical interference surfaces cooperating with corresponding cylindrical interference surfaces in the surrounding bore, said interference surfaces of the stud having different diameters such that the diameter decreases towards the free end of the stud.

7. A screw rotor machine as defined in claim 6, having annular grooves in the rotationally symmetrical surfaces of said stud and further channels having openings located such that said channels open into said grooves.

8. A screw rotor machine as defined in claim 7, in which the channels leading to the different pairs of cylindrical interference surfaces are mutually totally separate.

9. A screw rotor machine as defined in claim 1, in which in each coaxial group the rotors have the same pitch diameter.

10. A screw rotor machine as defined in claim 9, in which the rotors of one set have a smaller peripheral flank clearance than the peripheral flank clearance in the other sets of cooperating rotors.

11. A screw rotor machine as defined in claim 10, in which the rotor set having the smallest peripheral flank clearance comprises a rotor having a central bore.

12. A screw rotor machine as defined in claim 10, in which the peripheral flank clearance of the first mentioned set is about twice that of the latter set.

13. A screw rotor machine as defined in claim 8, in which the lands of two coaxial interconnected rotors have oppositely directed helices and that the thrust bearings of the rotors are of antifriction type and are adapted for zero clearance.

References Cited by the Examiner UNITED STATES PATENTS 1,132,747 3/1915 Sundh 1031 18 1,677,980 7/1928 Montelius 103128 2,441,771 5/1948 Lysholm 103-128 2,459,709 1/1949 Lysholm 230143 2,477,004 7/1949 Paget 230-143 2,504,230 4/1950 Smith 230-143 2,622,787 12/1952 Nilsson 230143 2,659,239 11/1953 Nilsson et a1 103128 2,683,994 7/1954 Whitfield 103--l28 2,775,204 12/1956 Batten et a1. 103118 2,975,963 3/1961 'Nilsson 230143 3,074,624 1/1963 Nilsson et al 230-143 MARK NEWMAN, Primary Examiner.

W. I. GOODLIN, Assistant Examiner. 

1. A SCREW ROTOR MACHINE FOR AN ELASTIC WORKING FLUID COMPRISING COAXIAL ROTORS JOURNALLED IN A HOUSING IN PARALLEL GROUPS AND PROVIDED WITH HELICAL LANDS AND INTERVENING GROOVES HAVING WRAP ANGLES OF LESS THAN 360*, ONE ROTOR IN A GROUP COOPERATING WITH A ROTOR IN ANOTHER GROUP WITHIN EACH SET OF AXIALLY EQUALLY LOCATED ROTORS OF THE DIFFERENT GROUPS AND ONE OF SAID COOPERATING ROTORS AT LEAST IN ONE SET BEING OF MALE ROTOR TYPE AND THE OTHER OF FEMALE ROTOR TYPE, THE HOUSING HAVING AT LEAST ONE WORKING SPACE PROVIDED WITH LOW PRESSURE AND HIGH PRESSURE PORTS AND BARREL AND END WALLS SEALING ENCLOSING ROTORS PROVIDED THEREIN, ONE ROTOR AT LEAST IN A FIRST COAXIAL ROTOR GROUP COMPRISING AT LEAST ONE AXIALLY PROJECTING STUD, AT LEAST ONE FURTHER ROTOR HAVING A CENTRAL BORE SURROUNDING AND CONNECTED WITH SAID STUD BY THE INTERFERENCE BETWEEN AT LEAST ONE PAIR OF ROTATIONALLY 